1. Field of the Invention
The present invention relates to an optical wireless communication system for performing wireless communication in free space by using light, and a transmitter and receiver used for this system.
2. Related Art
In order to realize movable communication with multimedia functions, radio communication with higher transfer rates is required, and there is a need to develop new frequencies. In the field of radio waves, development is proceeding which aims at realizing radio communication with submillimeter and millimeter waves. On the other hand, there is also an expanded utilization of infrared rays in the field of radio communication, although they are not regulated by the law as radio waves.
Using infrared rays, namely a wide band which is not under restriction as radio waves in the field of optical wireless data communication, the provision of high speed data communication may be possible. As light is characterized by not penetrating non-transparent objects such as walls, it is suitable for use in wireless LANs of individual room or for short distance data communication. At present, the most typical wireless communication system using infrared rays is the IrDA (Infrared Data Association) system. This system is composed of an infrared light-emitting diode and a light-acceptor, and realize data exchange at high speeds from 115.2 kbps to 4 Mbps. The communication distance is short, namely 1 m or less, but the greatest characteristic is that it can provide wireless data communication at low costs.
In the future, the demand will grow for an optical wireless data communication system with a larger transmission capacity and greater communication distance. However, when using light-emitting diodes as the light source, the light emitted from the light-emitting diode has a wavelength width of 100 nm or more, so the effective utilization of the band is ineffective. Furthermore, due to the long life time of carriers in LEDs, a modulation exceeding 100 MHz is difficult. In order to solve these problems, it is effective to use semiconductor lasers as the light source. By using a semiconductor laser, a wavelength width of 1 nm or less is easily obtained, and modulation to 1 GHz or more is principally also possible. However, there is the problem that errors may be caused by crosstalk. As light is not subject to legal restriction as radio carrier waves, it can be used without restriction, but the disadvantage may arise in that optical wireless equipment which utilize the same wavelength will mutually interfere. For example, existing optical wireless data communication systems such as the IrDA system utilize wavelengths from 850 nm to 900 nm, as their peak wavelengths. Even if a communication device using semiconductor lasers which provides high speed transmission and long communication distances were realized, using any of the wavelengths from 850 nm to 900 nm would lead to interference with the IrDA system. As the IrDA system is widely used with existing computers, the interference must be avoided in practice, even though it may not be a legal problem.
In order to prevent the interference, the wavelengths need to be selected so as not to overlap with the wavelengths already in use. For example, in order to avoid interference with the IrDA, it is effective to use a wavelength of 1 .mu.m or more. However, the problem is the related cost. In order to utilize wavelengths of 1 .mu.m or more, there is the need to use an InGaAsP mixed crystal formed on an InP substrate as the transmitting semiconductor laser. This substrate is more expensive than a GaAs substrate and increasing the diameter is difficult. Therefore, this system is less cost effective than those systems which use wavelengths of 900 nm or less which can use the GaAs substrate. Furthermore, PIN photodiodes for receiver can be cost problems as well. The reason is that PIN photodiodes made of silicon are limited to using only wavelengths of 1 .mu.m or less. It has no sensitivity in wavelengh range exceeding this limit. With wavelengths exceeding 1 .mu.m, a PIN photodiode made of InGaAs formed on an InP substrate is necessary. As material and production costs for this element highly exceed those made of silicon, a light detector whose area is comparatively large will cause a large cost difference.
Accordingly, the realization of high-speed optical wireless data communication utilizing semiconductor lasers is difficult due to the increased cost in making an approach of using long wavelengths of 1 .mu.m or more to prevent interference with existing optical wireless data communication systems.
There is also an additional problem in providing an optical wireless data communication system at low costs. A clock synchronized with the received data is necessary to reproduce the received data from the received light. Conventional optical wireless data communication systems extract the clock via a PLL (Phase Locked Loop) circuit from an electric signal obtained by photoelectrically converting the received light. The stable operation of this PLL circuit required high precision circuits and stable power sources, which became factors for raising the cost. It also required the modulation into RLL-type (Run Length Limited: limited number of bits in which "1" and "0" follow successively) modulated codes during the transmission so as to allow easy extraction of the clock from the received light. The modulation and demodulation circuits were other factors for raising the cost.